[windows] Enable tests for LLDB in Swift CI

[weliveindetail]

#76894 prepared the toolchain build to run tests for LLDB in Swift CI. Here we just turn it on. In case we clog Swift PR testing, we can easily revert this one without loosing all the infrastructure.

[weliveindetail]

@swift-ci please smoke test

[weliveindetail]

The previous PR test https://ci-external.swift.org/job/swift-PR-windows/35709/ took 3h52min
The test for this PR https://ci-external.swift.org/job/swift-PR-windows/35710/ took 4h5min

As expected, LLDB added 5min to the llvm-project tests. Where are the remaining 8min coming from? Not clear. The log of the build step times below doesn't explain this gap. The accumulated times are 3:30:37 vs. 3:22:42. These 3min are in range of natural fluctuation, but we still miss 5min of extra runtime. We might want to add extra time tracking.

[weliveindetail]

If the Windows arm64 bot reports new failures, we want to add them to utils/windows-llvm-lit-test-overrides.txt. I am keeping an eye on https://ci-external.swift.org/job/swift-main-windows-toolchain-arm64/919/ and I hope it's done before the end of my day 🤞

Edit: swift-main-windows-toolchain bot don't run the LLDB tests right now, because their invocation is hardcoded. We have to wait for the maintainer to enable it. My gut feeling is that we should keep them enabled in PR-testing as long as there are not complaints about flakes. So far, random sampling didn't show any, but this way of monitoring is fragile.